The present invention relates to a controller suitable for use with image forming apparatus such as copying machines, printers and the like.
In a conventional image forming apparatus such as a copying machine, printer and the like, a sequence controller circuit for controlling loads such as motors, solenoids and the like and controlling turn on and off low voltage power sources, exposure lamp power sources, and charging power sources is mounted on a circuit board separately from the power sources.
Such a power source uses switching regulators which require a control circuit constructed of an error amplifier and a pulse width modulation (PWM) circuit, the former comparing a fraction (error signal) of the power source output with a reference signal, and the latter controlling the power source output in accordance with an output from the error amplifier. The error amplifier and pulse width modulation circuit are made of discrete circuits, or made of a power source control IC integrating both circuits in one chip.
In order to simplify the circuit for stabilizing a power source, a method has been proposed wherein stabilizing control is carried out by a sequence controller microcomputer using programs.
A conventional apparatus having necessary circuits formed on different circuit boards is difficult to be made compact.
Use of an error amplifier leads to a complicated circuit arrangement because the amplifier must satisfy the off-set voltage and frequency characteristics necessary for controlling the power source. Further, since a large capacity is interconnected between inputs and outputs for phase compensation, the input and output terminals are required to be mounted outside of the circuit board. Furthermore, it becomes necessary to use a large number of cells to deal with electrostatic problems, so that the error amplifier is difficult to be mounted together with digital circuits including a CPU.
Use of microcomputers and programs for controlling a power source necessiates high speed microcomputers and high speed and precision A/D converters. Such programs become complicated, and the peripheral analog circuits of microcomputers such as driver circuits, level conversion circuits and the like are made of discrete circuits, resulting in a hardship on compactness and simplification. Therefore, controlling a power supply by using microcomputers and programs is still not realized in practice.
In a conventional image forming apparatus such as a copying machine, printer and the like, various circuit elements have been provided independently and separately, the circuit elements including a sequence controller circuit whose main elements are a microprocessor for controlling the entirety of print sequence, a DC power source, and a high voltage power source such as an exposure lamp power source and charger. Therefore, there has been limits in making the apparatus compact and inexpensive.
In order to form such circuit elements on one board, it has been proposed to integrate microprocessors, RAMs, ROMs, digital peripheral circuits, A/D converters, D/A converters, pulse width modulation circuits (hereinafter called PWM circuits) for controlling power sources, and the like, all in one chip.
However, if circuit elements are integrated in one chip without any particular consideration, the circuit scale, particularly of PWM circuits becomes large, resulting in a large chip area with less cost effectiveness.
The following problems have been associated with a conventional PWM circuit.
FIG. 36 is a schematic circuit diagram showing the main portion of a conventional PWM control unit. This control unit is provided with an error amplifier 1 and an oscillator 2, the error amplifier comparing a detected output voltage with a set value and amplifying a difference therebetween. Obtained from a diode D1 is a DC level S10 which is used as a control level for controlling the output voltage. The oscillator 2 outputs a triangular wave signal S12 whose frequency is determined by a resister R1 and capacitor C1. The signal S12 is compared with the DC level S10 at a comparator 3. Switching elements are controlled to turn on and off in accordance with an output S13 from the comparator 3, to thus regulate the output voltage of a power supply. The maximum duty of PWM is determined by resistors R2 and R3. FIG. 37 shows the waveforms S10, S12 and S13. In FIG. 36, D2 represents a diode, and R4 represents a resistor.
The triangular signal S12 outputted from the oscillator 2 of the above PWM control unit is an analog signal. Therefore, there arise some problems in that disturbances of waveforms thereof due to noises are large, resulting in poor frequency precision, poor precision of upper and lower limits of the triangular wave, and hence large error in duty ratio settings.